Coreless transformers are transformers with a primary winding and a secondary winding without any winding core. The primary and secondary windings in transformers such as these are, by way of example, in the form of planar windings, which are dielectrically insulated from one another and, for example, are arranged on or in a semiconductor body. One such coreless transformer is described, for example, in DE 102 32 642 A1.
Coreless transformers are used as the potential barrier or as DC isolation in data transmission paths, for example, in data transmission paths for driving semiconductor switches, in particular high-side switches in half-bridge circuits, or data transmission paths for data transmission in industrial control systems.
The primary winding of a coreless transformer has a time constant which is governed primarily by the cross section of the winding and by the coil diameter of the winding—which runs in a spiral shape in the case of planar windings. This time constant is the quotient of the inductance and resistance of the primary winding. A voltage on the secondary winding decays with this time constant when a step-function exciter signal is applied to the primary winding. For data transmission via a coreless transformer such as this, a drive circuit on the primary side must produce a drive signal whose flanks have rise periods and fall periods which are considerably shorter than this time constant. Furthermore, an evaluation circuit on the secondary side must be able to identify voltage pulses on the secondary side which are decaying with this time constant. The speed of the transmission and reception circuits which are involved with the data transmission on the primary side and secondary side is thus also determined significantly by the parameters of the windings. In the case of windings which are integrated in or on a chip, the winding parameters significantly govern the required chip area.
By way of example, differential transmission methods are used for signal transmission via data transmission paths with coreless transformers such as these. In methods such as these, information that is contained in a two-value signal is transmitted by means of signal pulses which characterize rising and falling flanks of the signal to be transmitted. In order to distinguish between rising and falling flanks, the transmitted signal pulses may have different amplitudes or mathematical signs, or the signal pulses for rising and falling flanks may be transmitted via different channels. The two-value signal is recovered from the transmitted pulses at the receiver end of the transmission channel by means of a receiving circuit with a suitable detection and demodulation arrangement.
There is a risk in the transmission paths that have been explained of disturbance signals, for example disturbance signals resulting from electromagnetic interference, being injected into the transmission path, which have the same shape and amplitude as a transmitted information pulse and which can lead to transmission errors. When a semiconductor switch is driven via a transmission path such as this, unidentified disturbance pulses can lead to an undesirable switching state of the switching element.
In order to reduce the robustness of a differential transmission method to disturbance pulses, U.S. Pat. No. 4,027,152 and U.S. Pat. No. 6,262,600 each disclose a differential signal transmission method in which flanks of a signal to be transmitted are converted to signal pulses, and in which these signal pulses are in each case repeated periodically in order to make it possible to identify and correct transmission errors caused by disturbance influences. This method has the disadvantage of comparatively high power consumption which results from the periodic repetition of the transmitted signal pulses.
Differential signal transmission methods are known from DE 102 44 186 A1 or DE 102 29 860 A1. In these methods, the time information about rising flanks of a two-value signal is transmitted in the form of pulses via a first transmission channel, and the information about falling flanks of this signal is transmitted in the form of pulses via a second transmission channel. In these known methods, both transmission channels are monitored for disturbance signal detection, in order to retransmit the most recently transmitted payload signal pulse on detection of a disturbance signal.
U.S. Pat. No. 6,525,566 B2 describes a further transmission method via a data transmission path with two coreless transformers. In this method, rising/falling flanks of a two-value signal are in each case converted to radio-frequency signals, and are transmitted via one of the two transmission channels.
If the primary winding is driven by means of square-wave signals, a signal, that is to say a voltage or a current, on the primary winding can rise only as quickly as an associated driver circuit can produce the current. Particularly if the supply voltage is low, parasitic inductances can significantly influence this rise time.
Furthermore, electromagnetic disturbance pulses are produced to a not inconsiderable extent both in the most recently explained method and in the previously explained method, in both of which square-wave pulses for transmission and for driving the primary winding are produced on the primary side.